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In the paper is presented a numerical analysis of optical sensors based on gradient index multimode interference structures (MMI) made by sodium-potassium ion exchange is presented. Variations of external propagation conditions change the mode properties and as a result the modal fields interference pattern and the signal distribution at the output. Three sensor configurations are considered. In the first case multimode section is covered by dielectric layer which change the real part of the refractive index during its contact with measured analyte. The second configuration use the additional thin buffer layer of a high refractive index placed between the core of a multimode section and a dielectric sensing layer which increases the energy of modes in the active region. A third configuration is based on the MMI section with cladding layer that changes the refractive index and extinction with changes of the concentration of the measured analyte.